Investigation of the genetic transmission of the ecotropic and xenotropic classes of murine leukemia viruses in inbred mice has led to the chromosomal mapping of 9 distinct loci. Studies with related laboratory mouse strains and Asian mice have been undertaken to describe the stablity and wild mouse origin of specific loci and to identify additional genetic factors which alter in vivo virus expression. Most recently these studies have shown that the ecotropic V locus of CBA/J mice is allelic with that of several related strains. Other crosses are being used to describe the genetic basis for the great differences in induction efficiency shown by apparently allelic loci in DBA and SEA mice. The major xenotropic V locus of virus positive laboratory strains (Bxv-1) has now been identified in the Japanese mouse, M.m. molossinus. Finally, studies on the transmission of the endogenous B-type mouse mammary tumor virus in sexual crosses has shown that 3 unrelated mouse strains carry MMTV-related proviruses very near the MuLV locus Bxv-1.